The field of force and the form of the carbon dioxide molecule. - Part II. The variation of the viscosity of carbon dioxide with temperature

The variation of viscosity with temperature provides another method of investigating the forces between gas molecules. Chapman has obtained the following expression forn the coefficient of viscosity of a gasn 0·499mc ¯/√2πσ2 wherem = mass of molecule,c ¯ = average velocity, and σ = molecular diameter. Asc ¯ is proportional to the square root of the absolute temperature T, it follows that the value ofn must be proportional to √T for a gas in which the molecules are rigid elastic spheres. For a gas in which a decreases with rise of temperature,n ) will increase more rapidly than this as the temperature rises. The variation in the value of a will depend upon the law of force between molecules. Our experiments on thermal diffusion in carbon dioxide mixtures led to the conclusion that a definite change occurs in the field of force of this gas at about 145° C. This change should influence the viscosity of the gas, and conse­quently should be demonstrated by its effect on curves showing the relation betweenn and √T. We may expect either (a ) a discontinuity in these curves owing to a sudden change in the effective diameters of the molecules, or (b ) a singularity at a point in the curve where the general slope changes as a result of the change in molecular hardness.